1. Field of the Invention
This invention relates to a liquid crystal composition, particularly to a liquid crystal composition having an antiferroelectric phase (chiralsmectic C.sub.A * phase, hereinafter abbreviated to "SmC.sub.A * phase"), more particularly this invention relates to an antiferroelectric liquid crystal composition suitable for use for a liquid crystal indication element.
2. Description of the Related Art
Recently, liquid crystal displays have become widely used as indication elements, due to their reduced thickness, light weight, low power consumption etc. However, most of these displays use TN (Twisted Nematic) type display comprising a nematic liquid crystal. Since the operation of TN displays is based on the anisotropy of the dielectric constant of the liquid crystal, the response speed is slow, and improvement is required.
In contrast, liquid crystal devices comprising chiralsmectic C phase (hereinafter abbreviated as "SmC* phase") liquid crystals, which are ferroelectric and were discovered by Meyer et. al., have high response speeds and memory characteristics. Thus, in order to utilize these characteristics, application of these ferroelectric liquid crystals to displays have been intensively researched. However, the good orientation and memory characteristics required for this indication method are difficult to realize in practice. Many problems remain to be solved, such as sensitivity to external shocks, etc.
On the other hand, recently, an antiferroelectric phase (hereinafter abbreviated as "SmC.sub.A * phase") has been discovered by Chandani et. al. which shows three stable states on the lower temperature side of said SmC* phase. This antiferroelectric liquid crystal shows a thermodynamically stable phase wherein dipoles are arranged in antiparallel in every adjacent layer, and exhibits an electric field-induced phase transition between the antiferroelectric phase and the ferroelectric phase which is characterized by a clear threshold and double hysteresis in response to applied voltage. Investigations on indication methods utilizing this switching behavior have already begun.
Liquid crystal compounds having an antiferroelectric phase are disclosed in Japanese Unexamined Patent Publication (Kokai) Nos. 1-213390, 1-316339, 1-316367, 2-28128 etc., and their number continues to increase as new compounds are published. However, many of the antiferroelectric liquid crystals that have been manufactured until now have high melting points and their antiferroelectric phase occurs at temperatures far higher than room temperature. Therefore, it is difficult to manufacture a liquid crystal element using a single antiferroelectric liquid crystal compound, and usually, several or more antiferroelectric liquid crystal compounds must be mixed to obtain useful physical properties.
When the switching characteristics of an antiferroelectric phase are to be applied to a liquid crystal display, e.g. display for cars, the liquid crystal compositions used must have a stable liquid crystal phase over a temperature range including room temperature. Particularly, a technique for preventing crystallization of liquid crystals at lower temperatures is required. However, conventional liquid crystal compositions have narrow liquid crystal temperature ranges, and the crystallization temperature thereof is relatively high. Hitherto, in order to decrease the crystallization temperature, a compound having particularly low crystallization temperature has been blended with liquid crystal compositions, or the number of the components of a liquid crystal composition has been increased, for example, to 10 to 20 components. However, compounds having a low crystallization temperature have a small tilt angle and often have a high threshold temperature. On the other hand, the use of multi-component compounds causes problems in balancing the properties of the liquid crystal composition.